Carbon nitride such as β-C3N4 having the same crystalline structure as β-Si3N4 is one of most advanced and attractive material in recent years. β-C3N4 is expected to have a bulk modulus as high as 420-560 GPa which is comparable to that of diamond of 443 GPa. It also has an expected shear modulus as high as 300-400 GPa corresponding to that of boron nitride. See, A. Y. Liu and M. L. Cohen, Phys. Rev., B41:10727 (1990). By virtue of these properties, carbon nitride is expected to be highly valuable as surface protection films of cutting tools and also as materials having high thermal conductivity.
In the course of studying β-C3N4, a variety of other forms of carbon nitride including cubic carbon nitride and amorphous carbon nitride have been discovered. See, E. Kroke and M. Schwartz, Coordination Chem. Rev., 248:493 (2004). These new forms of carbon nitride are attracting increasing attention as well and include those having unique properties such as high hardness and high wear resistance or capability of varying band gaps.
Carbon nitride films have hitherto been produced by reacting carbon and nitrogen at a temperature above 2000° C. using plasma or laser beam. See, JP 11189472A, JP 2001232501A and U.S. Pat. No. 6,658,895B2. These methods require complicated and expensive apparatus and, therefore, make the cost of resulting products economically unacceptable.
H. Kawamura and Y. Ito reported in Journal of Applied electrochemistry, 30:571 (2000) a method for electrochemically depositing carbon films on a substrate using a molten salt electrolyte bath containing carbonate ion. The carbonate ion is reduced to deposit a carbon film on the surface of the substrate acting as cathode. As will be easily appreciated, this method per se is not applicable to deposit carbon nitride films on a substrate.
A need exists, therefore, for a novel method for depositing carbon nitride films on a substrate which can eliminate or ameliorate of the defects of the known methods while taking advantages of electrodeposition process.